Blender Git Loki

Fix accumulated brushes. Those would never completely fill canvas
because mask factor of brush never reached 1.0 due to it being
calculated by geometric series with factor less than one. Now simply add
new mask to the accumulated mask.

Note - it may be preferable to handle airbrushes like that too, since
they suffer from the same issue.

2D case will be handled separately, things look unnecessarily complex
here already, and I have totally forgotten to handle correct masking in
float case.